Monthly Climate Statement for October 2010

The Queensland Climate Change Centre of Excellence considers that there is an increased probability of above-median rainfall throughout most of Queensland for the remainder of spring and the coming summer season. 

The Centre’s understanding is based on the current and projected state of the El Niño-Southern Oscillation (ENSO) phenomenon and on factors which modulate the impact of ENSO on Queensland rainfall (for example the Pacific Decadal Oscillation).

As at October 1 2010, the Centre notes that:

The current La Niña climate pattern is now well-established in the Pacific (see the latest Bureau of Meteorology ‘ENSO Wrap-Up (PDF)*’) and is likely to persist throughout spring and summer.  In particular:

• The Southern Oscillation Index (SOI) remains extremely positive when averaged over the last month (September: +25.8), two months (August-September: +21.4), and three months (July-September: +20.5). 
• Observed sea surface temperatures in the key Niño 3.4 and Niño 4 regions (PDF)* remain much cooler than normal – typical of a well-established La Niña pattern.
• Associated with this La Niña pattern, the sea surface temperature gradient (west to east) across the South Pacific Convergence Zone (PDF, 622K, last updated 11:10AM, 8 December 2010)* (i.e. between eastern Australia and the Central Pacific) remains very positive (+1.75 ºC).  According to the Centre’s experimental SPOTA-1 scheme), a positive sea surface temperature gradient across this region is favourable for rainfall in Queensland. 
• As reported last month, the current La Niña phase has clearly ‘locked in’ and is therefore likely to persist through the coming summer. The strong likelihood of a La Niña pattern persisting through summer is also supported by global climate models.

The recent sea-surface temperature pattern in the North Pacific remains consistent with a ‘Cool Phase’ of the Pacific Decadal Oscillation (PDO).  The PDO modulates the impact of ENSO on Queensland rainfall particularly under La Niña conditions.  A cool phase of the PDO coupled with La Niña conditions is particularly favourable for rainfall in Queensland. These conditions are also usually associated with enhanced tropical cyclone activity in the Coral Sea, which is discussed in the Bureau of Meteorology’s Seasonal Outlook 2010-11 for Queensland and the Coral Sea. 

There are various approaches to developing probabilistic rainfall outlooks based on the information considered above.  These approaches tend to differ in terms of which components of the climate system are considered.  As such, each approach might convey a different outlook, particularly for specific locations.  However the ENSO and PDO signals have historically had the strongest impact on rainfall in north-eastern Queensland and the weakest impact in south-eastern Queensland.

The Centre produces two statistical climate risk assessment schemes:

• The Centre’s experimental SPOTA-1 scheme integrates the above sea-surface temperature information, including indices of ENSO and the PDO.  SPOTA-1 currently indicates a high probability of exceeding median rainfall across the state over the coming summer (November to March).
• The Centre’s SOI Phase scheme, which relies on the SOI, currently indicates a high probability of exceeding median rainfall across much of the state over the coming three-month period (October to December).

It is important that users understand the nature of such seasonal outlooks and take a long-term risk management approach to such information.  The above schemes indicate rainfall probabilities based on historical relationships.  Users should appreciate that if, for example, an outlook is for a 70% probability of above-median rainfall, this also means there is a 30% probability of below-median rainfall.  As such, users should also be aware that an increased risk of above or below-median rainfall in Queensland due to ENSO will not necessarily result in above or below-median rainfall occurring throughout the state (for example, see Australia’s Variable Rainfall poster (PDF, 1.0M, last updated 11:42AM, 24 June 2010)* or our archive of historical rainfall maps).

The QCCCE understands that each of the schemes may have its own particular following.  Although such schemes cannot provide outlooks with absolute certainty each year, those who follow a skilful scheme should benefit from doing so in the long-term.  Users should consider the historical track record of any scheme and such information is becoming increasingly available. For example, an historical archive of SPOTA-1 reports is available on the Long Paddock website.  Users should also consider the wide range of information available each month describing the current state of the ocean/climate system. 

ENSO influences other climate variables apart from rainfall (e.g. temperature, pan evaporation and vapour pressure). This means that the impact of ENSO on crop or pasture growth can be stronger than on rainfall alone. The impact of ENSO on pasture growth is also dependent on current pasture condition and soil water status. The Centre’s AussieGRASS model takes these factors into account in producing pasture growth seasonal probabilities.

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